So... in joining Kethlas in playing devil's advocate...
I happen to work at a PetCo, and while I agree that the condition the bettas were in was terrible, and that the particular employee in question was way out of line, I can relate as well. Kethlas did make a good point in saying that the cleaning of the betta cups is left to the Aquatics specialist. Along with most other duties required to keep ALL the tanks in the store in good condition. And that's REALLY time-consuming. There are general tasks that have to be done each day, and then other more intensive tasks scheduled 2-3 times a week. We can't do everything everyday. Many times, we're not even able to finish ALL of a particular day's tasks.
As for being able to change a betta's water cup in 7 seconds? Being generous, and giving them 30 seconds for each cup, that's still nearly an hour to change out 100 betta cups. And this doesn't count the time spent transporting the cups to the sink area from the display and then transporting them back and getting the next batch. Plus dealing with customers, who, as was very aptly stated, are either ignorant, rude, or a downright nightmare, and being on call to attend whatever other situation might arise, what might seem like a quick task can easily stretch out into hours. And, unfortunately, many times we do carry a lot more than just a hundred cups.
And yes, unfortunately, fish do die all the time. We do everything we can to prevent it, but let's face it, the stress of transit, plus the stress of adjusting to a new tank environment, plus the stress of a potentially aggressive tankmate, plus the stress of stupid people who tap on the glass even when we ask them not to, plus the stress of having nets in and out of their tanks all day, chasing them about is way too much for some fish.
And again, there are days where we simply don't have the time. We might get a rush of people and are forced to stop midway through a task, or we might be shortstaffed that day, or it might be truck day. And while we might not get as many deliveries as a grocery store, it was stated plainly that deliveries are received all the time. That means that the inventory is spread out through the week. We get one HUGE shipment per week and we usually have only 2 people available to work truck, three MAYBE if we're lucky. And even the truck people have to stop what they're doing to help out customers. As an example: one day, our truck shipment was late due to some kind of accident or something, and didn't come in until early, early morning two days later. (Truck arrives Tuesday, so Thursday morning) FOUR people got called in at ONE in the morning to help put away truck. Two more people came in to help at five. It was nine in the morning, opening time, when I left, and truck STILL wasn't done. Nearly, but not yet. And that was with no customers to deal with.
It's not that we don't care, because we do. I'm sure there are those that don't, but I'd say at least 90 percent of the people that work at PetCo work there because they actually like animals. At least that's been my experience. Personally, I actually enjoy my job, because I do love working with the animals, all of them, and I love helping people find the right pet for them and making sure they know how to care for it. And I'm sad every time I find a dead animal, whether it's a fish, or a lizard, or a rat, etc. I reiterate that we do try our best; we have a vet on call that we work with if one of our animals is sick. However, it's just not always possible to save each one. Sometimes the animals arrive that way and despite our best efforts, they don't make it.
Aside from that rant, I agree that the employee was unjustifiably callous. She should have at least informed the manager of the customer's concerns. She definitely should have at least removed the cup with the dead betta. And the remark of "they die all the time" was beyond uncalled for. She shouldn't have continued to face the dog treats. Her response to the your concern was ridiculously inappropriate and should definitely be addressed.
I do understand your anger as to the neglect (and flippant attitude!), and I most definitely agree that it should never have gotten to that point in the first place, but I can also see how it could unfortunately get that way.